Start up problem in XP

Just in the last couple of days when I switch on my computer it fires up ok but an error message comes up on the screen it is three bars one red one green and one blue with the message something like 'Your monitor is working ok please check your graphic card or monitor cable' as I said that is not the exact message but close. I can get the computer to fire up normally if I switch it off and on a couple of times and sometimes by holding in the reset button as I switch on.

Tonight it started with the error message and after switching off and on a couple of times it then refused to start at all no power going to the m/b or from the PSU I removed the PSU and tried in another system it was ok I then put it back in my computer switched it on and it booted up first time so the fault is intermittent but is it a m/b,PSU,graphics card or switch problem?